Both wurtzite-and zincblende-GaN films have been grown on sapphire and MgO substrates, respectively, and examined by photoluminescence and x-ray analysis. GaN films were grown on suitably prepared Al2O3 and MgO substrates by molecular beam epitaxy employing a rf plasma discharge, nitrogen free radical source. The wurtzite-and zincblende-GaN films exhibited dominant near band-edge emission, the nature of which will be compared and contrasted for both phases in this paper. X-ray diffraction data for both phases will also be discussed.

The optical absorption near the fundamental absorption edge in GaN thin films grown on sapphire substrates is studied as a function of temperature. The absorption band edge was determined from either the energy position of the exciton line in samples grown by metalorganic chemical vapor deposition technique, or from the first derivative of the absorption spectra in samples grown by molecular beam epitaxy technique. The band edge energies determined in the temperature range of 13 – 300 K were fitted with Varshni empirical relationship: Eg(K) = Eg(0) – α T2/(T + θD) and with the expression: Eg(K) = Eg(0) – κ/[exp(θE/T) – 1]. It is found that Eg(0), α, θD, and θE to be sample-dependent, which suggests that defects and dislocations significantly affect the optical band edge in GaN.

Using picosecond time-resolved photoluminescence we have studied the decay time of excess carriers in GaN epitaxial layers over a wide range of temperatures from 4 K up to 400 K. At low temperature, a thermal dissociation of donor-bound excitons is observed. At higher temperatures up to room temperature, the luminescence decay at moderate excitation is governed by trapping of photogenerated electrons in ionized shallow donor levels. Using measured luminescence intensities to determine the quantum efficiency, we obtain the radiative lifetime of free excitons from low temperature up to room temperature. We use these data to determine the radiative recombination coefficient and the interband momentum matrix element.

Picosecond Raman spectroscopy has been employed to study electron-phonon interactions in the wide bandgap semiconductor GaN. An ultraviolet picosecond laser with photon energy ћω= 4.36eV was used to excite electron-hole pairs in an undoped bulk GaN. The relaxation of these high energy electrons and holes were used to interrogate electron-phonon interactions. We have found that electrons thermalize toward the bottom of the conduction band by emitting primarily longitudinal optical phonons. Our work demonstrates that the Fröhlich interaction is much stronger than the deformation potential interaction in wurtzite GaN.

A luminescence band centered around 3.42 eV is commonly observed in GaN. Its appearance has been tentatively assigned to an oxygen donor level. Stimulated laser activity has also been reported at this energy. We present a study of this band in GaN grown by high temperature vapor phase epitaxy (HTVPE). The high quality of this material, with an excitonic line width as narrow as 3 meV, allows us to distingish four different peak positions of this luminescence band. They appear at 3.4066, 3.4121, 3.4186, and 3.4238 eV (T = 6 K). Within the experimental error the lines exhibit an equidistant spacing of 6 meV. They show a pressure behavior similar to shallow levels described by effective mass theory. We discuss our results in the context of incorporation of oxygen and structural defects in GaN.

We performed spatially-resolved photoluminescence and Raman experiments on the substrate interface region of wurtzite GaN layers. We found that the broad ”yellow” photoluminescence band is strong only near the interface. Our investigations reveal that both the substrate interface and a region of structural reorientation of the layer near the interface act as source of the photoluminescence. The Raman-scattering experiments show that at least a portion of the GaN layer near the substrate interface is oriented in such a way that the c-axis of the layer is parallel to the substrate interface. At a distance about 30 µm away from the interface the layer reorients by turning the c-axis by 90° into a direction perpendicular to the substrate interface.

We report on the excitation wavelength and power dependence of LPMOCVD grown GaN photoluminescence (PL). A mode-locked Ti:Sapphire laser that is doubled to provide a tunable wavelength range of 350-500 nm is used to pump the GaN samples. Special attention has been paid to the “yellow” (∼567 nm) luminescence. Different power dependence characteristics of the yellow emission are observed when pumping above and below the bandgap. A measurement of the photoluminescence excitation spectrum of the yellow peak (∼567 nm) is also performed and this spectrum shows a peak at room temperature at 3.36 eV (369 nm), an additional broad peak at 77 K and only the broad peak at 7 K. An excitation model is used to explain the behavior of the yellow emission, suggesting the neutral donor serves as a key species for the yellow emission.

Recombination processes and their dynamics are selectively studied (a) near the interface betweeen a 400μm thick GaN epilayer and its Al2O3 substrate and (b) in the structurally relaxed regions near the surface of this sample. Strong radiative excitonic recombination is observed in the relaxed regions. However, the recombination dynamics of free and shallow-bound excitons here are strongly influenced by shallow and deep defects. Near the substrate the presence of dislocations suppresses radiative recombination of free and shallow bound excitons. Deeper emissions appear which we attribute to excitons deeply bound to dislocation-related defects. They exhibit ps-recombination dynamics effected by strong nonradiative contributions.

We report low temperature photoluminescence (PL) spectra related to a two-dimensional electron gas confined at a GaN/AlGaN heterointerface. The recombination between electrons confined in the bottom of the interface potential and photoexcited holes causes a broad PL emission about 50 meV below the bulk GaN exciton emission. A second emission, attributed to the recombination of electrons in the first excited level at the interface, is also observed close to the excitonic band gap in GaN. The data agrees with a self consistent calculation of the energy levels and the electron concentration at the interface. Similar PL data from a modulation doped AlGaN/GaN quantum well exhibit three PL emissions related to the 2D electron gas.

We present calculated longitudinal frequency and wavevector dependent dielectric functions of zincblende and wurtzite structure GaN using band energies and wavefunctions generated in the framework of the empirical pseudopotential method. We discuss the anisotropy of the static dielectric function and find that our results are in satisfactory agreement with experimental data for |q| ₒ 0.

Detailed photoluminescence (PL) studies of high-quality MBE-grown single-crystal cubic and hexagonal GaN are presented. We identify free and bound exciton recombination. By means of a line-shape analysis, we quantitatively analyze our spectra, which were taken as a function of temperature (T = 4 - 300 K) and excitation density (Pex = 0.3 - 200W/cm2). We show the dominant recombination channel at 300 K to be free-excitonic in nature with an internal small-signal quantum efficiency of 6 · 10−3 for both cubic and hexagonal material. Based on a three-level model, activation energies for exciton dissociation are evaluated. Radiative (τrad ≈ 2 ns) and nonradiative lifetimes (τe ≈ 1μs, τh ≈ 20 ps) are determined, where in the latter case, electron and hole trapping are considered separately. Furthermore, we show that the dominant nonradiative recombination center, being a hole trap, saturates at Pex ≥ 20 W/cm2.

We study deep defects in AIN crystals in the near infrared by photoluminescence and compare the observed emissions with those in GaN. By below bandgap excitation with an Ar ion laser three no-phonon lines at 1.043 eV, 0.943 eV, and 0.797 eV were detected, which are caused by different residual transition metal contaminants. The weak emission at 1.043 eV and the intensive emission at 0.797 eV show A1N related phonon sidebands, whereas the emission at 0.943 eV has practically no phonon sideband. No hot lines could be detected for the emissions at 0.943 and 0.797 eV in temperature dependent measurements. We discuss possible identifications of the luminescence centers and the similarity between GaN and AIN in view of transition metals.

The yellow Luminescence in GaN centered at 2.2 eV has been studied in various epitaxial layers grown by MOVPE on sapphire and by the sandwich sublimation methode on 6H-SiC substrates. The photoluminescence and optically detected magnetic resonance results can be consistently explained by a recombination model involving shallow donors and deep donors.

We report the growth and photoluminescence characterization of GaN grown on different substrates and under different growth conditions using metalorganic chemical vapor deposition. The deep-level yellow luminescence centered at around 2.2eV is attributed to native defect, most possibly the gallium vacancy. The yellow luminescence can be substantially reduced By growing GaN under Ga-rich condition or doping GaN with Ge or Mg.

We have studied by photoluminescence (PL) and optically detected magnetic resonance (ODMR) un-doped, n-doped and p-doped thin wurtzite GaN layers grown by metal-organic chemical vapor deposition on sapphire substrates. From the PL data for free excitons an accurate value of the free A-type exciton binding energy and a more accurate estimate for the hole effective mass is deduced. The localization energies of the Mg and the Zn neutral acceptor bound excitons are found to be in good agreement with Haynes’ rule. A sharp emission line, assigned to free electron recombination at a 116 meV shallow acceptor, together with three additional weak zero-phonon-lines (ZPLs), assigned to distant donor-acceptor (DA) pairs, are reported for the first time. The chemical nature of this acceptor and that of three residual donors, inferred from the DA pair ZPLs, is discussed. The effects of strain in thin GaN layers on a dissimilar substrate like sapphire are emphasized with respect to the energetic position of narrow PL lines. The ODMR data obtained for undoped, Mg-doped and Zn-doped GaN layers provide insight into the recombination mechanisms responsible for the broad yellow (2.25 eV), the violet (3.15 eV) and the blue (2.8 eV) PL bands, respectively. The ODMR results for Mg and Zn also show that these acceptors do not behave effective mass like and indicate that the acceptor hole is mainly localized in the nearest neighbor shell surrounding the acceptor core.

We have studied the electronic and energetic properties of native defects, impurities and complexes in GaN applying state-of-the-art first-principles calculations. An analysis of the numerical results gives direct insight into defect concentrations and impurity solubility with respect to growth parameters (temperature, chemical potentials) and into the mechanisms limiting the doping levels in GaN. We show how compensation and passivation by native defects or impurities, solubility issues, and incorporation of dopants on other sites influence the acceptor doping levels. The role of hydrogen in enhancing the p-type doping is explained in detail. We also discuss the mechanisms responsible for the experimentally observed limitation of the free-carrier concentration in p-type GaN.

Electrically detected magnetic resonance (EDMR) is used to study recombination processes in two types of gallium nitride light emitting diodes: in m/i/n/n+- and InGaN/AlGaN double-heterostructure devices. In the MIS-diodes, two resonances at g=1.96 and 2.00, corresponding to the effective mass donor and a deep defect are observed at room temperature. At low temperatures, an acceptor-related resonance at g=2.06 is visible as well. After current degradation, the spectra are dominated by the defect resonance, indicating that the creation of this defect is responsible for the decreased electroluminescence efficiency. In the double-heterostrucrure devices, EDMR can only be observed below 60 K showing the g=2.00 defect resonance. The same defect resonance is also observed in conventional electron spin resonance experiments under illumination (light-induced ESR).

Photoluminescence (PL) and optically-detected magnetic resonance (ODMR) experiments have been performed on undoped GaN epitaxial layers grown on 6H-SiC substrates. The defects observed in these films are compared with those found from previous ODMR studies of undoped GaN layers grown on sapphire substrates. Strong, sharp donor-bound exciton bands at 3.46 -3.47 eV and weak, broad emission bands at 2.2 eV were observed from several 0.7 and 2.6 μm-thick films. In addition, fairly strong shallow donor - shallow acceptor (SD-SA) recombination with a zero-phonon-line at 3.27 eV was found for GaN layers less than 1 μm-thick. The first observation of magnetic resonance on this SD-SA recombination from undoped GaN is reported in this work. Two magnetic resonance features attributed to effective-mass (EM) and deep-donor (DD) states were detected on the 2.2 eV emission bands from all the GaN/6H-SiC films. These resonances were observed previously on similar emission from undoped GaN layers grown on sapphire substrates. The same EM donor resonance, though much weaker, was also found on the SD-SA recombination. However, a resonance associated with shallow acceptor states was not observed on this emission. The weakness of the donor resonance arises from the weak spin-dependence of the recombination mechanism involving spin-thermalized shallow acceptors. The absence of an EM acceptor is due to the broadening of the resonance through the spreading of the acceptor g-values by random strains in these films.

Magnetic resonance techniques are used to study the recombination processes in GaN-based light emitting diodes (LEDs). Electrically-detected magnetic resonance (EDMR) and electroluminescence-detected magnetic resonance (ELDMR) results on InGaN/AlGaN double heterostructures are presented for blue and green LEDs. In either technique our signals are dominated by a broad feature that we ascribe to a deep Zn-related acceptor. Our ELDMR measurements show that this is associated with the blue or green emission. Our EDMR measurements resolve a second center that is tentatively identified as a deep donor trap.

A comparison between 300 K electron transport data for state-of-the-art wurtzite GaN grown on sapphire substrates and corresponding theoretical calculations shows a large difference, with experimental mobility less than the predicted mobility for a given carrier concentration. The comparison seems to imply that GaN films are greatly compensated, but the discrepancy may also be due to the poorly known values of the materials parameters used in the calculations. In this work, recent analysis of transport and SIMS measurements on silicon-doped GaN films are shown to imply that the compensation, NA/ND, is less than 0.3. In addition, the determination of an activation energy of 34 meV in a GaN film doped to a level of 6×1016 cm−3 suggests either that a second, native donor exists in the doped films at a level of between 6×1017 cm−3 and 1×1017 cm−3, or that the activation energy of Si in GaN is dependent on the concentration, being influenced by impurity banding or some other physical effect. GaN films grown without silicon doping are highly resistive.